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38-620 Tuning Question - Printable Version

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38-620 Tuning Question - Jamie - 02-25-2014

I just finished recapping a 38-620. The radio is working fine and it sounds great. The only problem is when I try to tune below 650. From 650 - 550 I get loud noise while tuning. It only happens when I am moving the dial, not when it is still. In other words, as I am moving the dial from 650 to 550, the noise is loud. But if I stop, say at 600, the noise stops too and any station at the freq will play fine. The noise sounds like someone blowing into a microphone. The noise is not affected by the volume control - it is extremely loud even if the volume is down.

I initially suspected the tuning cap had some dirt, or possibly a bent plate, but everything looks good. I sprayed it down with contact cleaner and blew it out with canned air. No help.

Any ideas?

Schematic - http://www.nostalgiaair.org/PagesByModel/254/M0013254.pdf

RE: 38-620 Tuning Question - skyscraper - 02-25-2014

I read somewhere that little conductive 'hairs' can form between the plates, run a business card between them and see if it helps.

RE: 38-620 Tuning Question - exray - 02-25-2014

Apart from the plates there is also the question of contact to the rotor of the cap. Make sure the connection, usually a spring finger on the shaft, is clean and making good contact.

RE: 38-620 Tuning Question - Jamie - 02-25-2014

Okey dokey... thanks guys. I'll try these ideas tomorrow. One other problem popped up. I left the radio turned on all day. After about 5 hours of playing, the transformer started buzzing like a vibrator. It stays nice and cool, it just buzzes like a little motor. I suppose there is something loose inside it, and I need to rebond it? I tried cranking the bolts tighter, but it didn't help. Any ideas on this?

RE: 38-620 Tuning Question - morzh - 02-25-2014

See if the voltage rising above a certain point (like 110V AC for instance) makes it buzz. It might dpend on the time of the day also.

RE: 38-620 Tuning Question - Joeztech - 02-25-2014

I have seen cases where the internal bobbin that holds the primary and secondary windings was loose enough that they would vibrate. Various techniques can be used to solve this. One of the easiest I saw was to shove a piece of a flat wooden toothpick into the area where there was an air gap visible. Some have used lacquer to stabilize vibrating windings. If there is an insulating gap material between leaves of the transformer that has deteriorated and fallen away that would be another possible area of vibration. I suitable piece of fish paper or other insulating material that can stand high temperatures would be the solution there. You might be able to use some Teflon tape for that situation.

Joe

RE: 38-620 Tuning Question - jerryhawthorne - 02-26-2014

PuhPow, in your first statement that the "microphone like noise" was not controllable by the volume control, loud at any setting seems to indicate to me something in the AF amp area or the volume control is not working right? Why at only one end of dial is perplexing to say the least.
I'm confused, as usual.
Jerry

RE: 38-620 Tuning Question - Jamie - 02-26-2014

Thanks guys. I had to work today, so I never got time to mess with the radio. I probably won't get back to it until Friday morning. I'll report back then.

Jerry, I agree with you. As I turn the dial, from high to low, the radio works great - until the dial hits 650. Then it makes a noise like someone blowing loudly into a microphone, at maximum volume. It hurts your ears. It does this as long as I continue to move the dial between 650 and 500. Volume control has no effect, the noise is LOUD. If I stop, say, at 600, the noise stops and if there is a station at that freq, it plays normally. Only happens between 650 and 500. It's perplexing. I'm going to try some of the tips suggested here.

RE: 38-620 Tuning Question - Mondial - 02-27-2014

The only thing I can think of is some sort of mechanical interference with a wire or other component at that certain position of the dial. Is there any wire or other conductive part close enough to touch the dial or gears of the tuning mechanism?

The part thats confusing is the fact that it is not volume dependent. This pretty much eliminates the tuning cap itself since any shorting of the plates would cause a noise in the RF and IF sections which would vary with volume setting. I think the tuning cap is isolated with rubber grommets, so possibly there a bad ground to the cap frame when tuned to the problem region of rotation.

I would look for a mechanical cause somehow related to the tuning cap mechanism itself.

RE: 38-620 Tuning Question - Jamie - 03-02-2014

Here is an update. I searched the net for information on buzzing transformers. I found a thread where an old timer said they used to remove the transformer, then tap each corner with a ball peen hammer. He said that would cure 90% of the buzzers and that they tried to do this when the customer wasn't looking!

Anyways, that's what I did. It still buzzes a little, but you can barely hear it. The "blowing into a microphone" tuning noise seems to have magically cleared up on it's own. Which is troublesome to me.... I like to find the cause of a problem so I know it will not return.

RE: 38-620 Tuning Question - gregb - 03-02-2014

Hey PuhPow, I had that same "blowing into a microphone" noise on my 45 on the shortwave band at the low end. I just discovered it to be the 6A7 tube.

Gregb